1. Field of the Invention
This invention is related generally to laminar flow control (LFC) panel construction and, more particularly, to a method that protects suction strips on perforated titanium sheets during fabrication processes.
2. Description of the Related Art
Perforated titanium sheets or skins are known for use in the construction of aerodynamic structures, including airfoils. The perforations are small holes in the airfoil surface through which suction is applied to achieve laminar flow control.
Precise control of areas of the perforated titanium sheet known as suction strips is required to achieve successful laminar flow control in flight environments. The exact widths of the suction strips must be maintained to achieve design suction porosities throughout the fabrication processes during which perforated skins are bonded to sub-structures including trapezoidal fluted composite structures. The skins or sheets are provided with electron beam perforated holes, typically with a 0.0025 inch diameter at the airfoil surface and a 0.025 inch center.
During bonding these perforated holes are exposed to harsh chemical etchant solutions. Moreover, when bonding composite pads with adhesive films to the perforated skins and substructures, high heating temperatures and pressures occur during the autoclave curing cycles which cause resin and adhesive to flow and wick into the perforated holes. While these fabrication processes are needed for strong bond lines, they can be detrimental to the laminar flow control panel suction strip porosities because of unpredictable margins and hole size changes. Many of the holes can become plugged during the bonding process or enlarged by harsh etchants. Any changes in hole geometry can potentially have an adverse effect on laminar flow control.
Varieties of masking materials and masking techniques are known for protecting a variety of surfaces. U.S. Pat. No. 3,046,175 to Bowman discloses a method of forming a curved surface on a honeycomb core using a plaster mold. A masking step uses mask and release layers which coat the surface of the mold prior to insertion of the honeycomb material into the mold. A non-water soluble "hot melt" material is pored into the other end of the honeycomb material and solidified. Afterwards, the plaster mold is dissolved so that the exposed portion of the honeycomb material, down to the mask and release layers, can be dissolved in an etching solution. Finally, the solidified hot melt is pulled out of the honeycomb material and the remaining mask layer is heated in a vacuum to soften the mask. Jets of air are then directed into the honeycomb to dislodge the mask. While Bowman teaches a mask and release layer, the masking material is not used to protect small diameter apertures such as those used for laminar flow control. Moreover, since the masking materials described in Bowman are thermoplastic instead of thermosetting, they are not used in a fabrication process which involves high temperatures used to cure composite structures.
U.S. Pat. No. 3,139,352 to Coyner discloses a masking method which uses a telomere of tetrafluoroethylene as a masking material. The masked surfaces are non-aerodynamic and include glass and bright metal fittings on automobiles and boats, handles, and drawer-pulls.
U.S. Pat. No. 3,212,949 to Thompson discloses an identification plate formed by masking a surface with a continuous perforated sheet. Strips of the sheet are removed in selected areas to enable painting or coating of the surface with material contrasting in color or reflectivity with that underlying the mask. The remainder of the mask is removed to expose an identification pattern.
U.S. Pat. No. 4,269,882 to Carrillo et al. discloses a perforated sheet bonded to a porous fibrous material on one side. The opposite side is masked by heavy paper using an adhesive which clings to the paper leaving the surface substantially free of adhesive when the heavy paper is removed. While the paper is in place, an anti-wetting solution is applied to the porous fibers material.
U.S. Pat. No. 4,587,186 to Nakamura et al. discloses a mask element for selective sand blasting to produce a pattern-engraved article corresponding to the pattern of the mask. The mask is formed on an unpatterned retainer film layer which in turn is formed on a support layer. The mask is sticky and adheres to both the surface of the object to be patterned and to the retainer film when pressure is applied against the support film. Subsequently, the support film can be stripped off without effecting adhesion between the retaining layer and the mask. Ultimately, the mask is removed by combustion or application of a solvent.
None of the above references address the problem of maintaining laminar flow control precise suction strip porosities and none teaches or suggests a method for masking a perforated aerodynamic surface.